The use of aqueous dispersions of photographic couplers and other hydrophobic photographically useful compounds is known in the art. Dispersions are suspensions of an oil phase in an aqueous phase, used to alter the character of photographically useful chemicals so that they can be incorporated into an aqueous gelatin matrix. The incorporated materials are generally high molecular weight, hydrophobic, crystalline materials such as couplers, dyes, Dox scavengers, and UV absorbers. Generally, dispersions of hydrophobic photographically useful materials (PUMs) in aqueous solutions are prepared by homogenization of a liquid organic phase containing a photographically useful material into an aqueous solution containing a hydrophilic colloid such as gelatin and, optionally, a surface active material. Methods of dispersion preparation of photographically useful chemicals are well-known in the art and have been described in, e.g., U.S. Pat. Nos. 2,322,027, 2,698,794, 2,787,544, 2,801,170, 2,801,171, and U.S. Pat. No. 2,949,360.
Processes for homogenization of liquid organic phases frequently include the use of low boiling or at least partially water miscible auxiliary solvents, which auxiliary solvent is subsequently removed after homogenization by evaporating volatile solvent or washing water miscible solvents. Such auxiliary solvents facilitate combining couplers and/or any other hydrophobic dispersion components in a mixed solution, so that a dispersion with an oil phase of uniform composition is obtained. The solvent also lowers the viscosity of the oil solution, which allows the preparation of small-particle emulsified dispersions. The use of auxiliary solvent may also be used to form a liquid organic solution of a PUM for forming a dispersion where no permanent solvent is desired in the final dispersion. The use of auxiliary solvent, however, also presents several potential difficulties in the preparation of photographic dispersions and elements. Auxiliary solvents can cause severe coating defects if not removed before the coating operation. Also, it is not possible, due to thermodynamic considerations, to remove 100% of the auxiliary solvent from the dispersion. This may cause other deleterious effects such as enhancing the solubility and movement of the PUM, or aid in crystallization. Further, the steps of evaporating volatile solvent from an evaporated dispersion and washing a chill-set, washed dispersion often leads to final photographic dispersions with variable concentration, so that careful analysis is necessary to determine the actual concentration of the photographically useful compound in the dispersion. Volatile or water-soluble auxiliary solvents present health, safety, and environmental hazards, with risks of exposure, fire, and contamination of air and water. The cost can be significant for the solvent itself, as can be the costs of environmental and safety controls, solvent recovery, and solvent disposal.
Alternatively, PUMs may be “directly” homogenized or dispersed into an aqueous solution in the substantial absence of any auxiliary solvent (i.e., absence of such solvents beyond trace or impurity levels). In one such direct dispersion process, the hydrophobic components desired in the dispersion, e.g., coupler and permanent coupler solvent, are simply melted at a temperature sufficient to obtain a homogeneous oil solution. This is then emulsified or dispersed in an aqueous phase, typically containing gelatin and surfactant. The direct process also yields a dispersion with a known concentration of the photographically useful compound, based on the components added, with no variability due to evaporation or washing steps. It is much less complex and less expensive because no volatile or water-soluble auxiliary solvent removal step is required. Further, since no auxiliary solvent is used, there are no associated environmental concerns. Additionally, the absence of auxiliary solvents in the dispersion forming step generally allows for higher concentrations of permanent organic phase (comprising the photographically useful materials and any high boiling permanent organic solvent) in the resulting dispersion. Processing times are shorter and material yields are higher. There are no dispersion quality issues related to the presence of residual auxiliary solvent in the finished dispersion. In addition, direct dispersions typically have a lower propensity for the formation of large oil droplets, which can cause physical defects in photographic film. For these reasons, the direct dispersion method is typically preferred over evaporated and washed processes, as it usually provides higher quality at lower cost.
While the direct dispersion process may in general be preferred for the above reasons, there are potential problems with the use of direct dispersions. Since there is no auxiliary solvent used, it is often more difficult to completely dissolve the photographically useful material and avoid dispersion crystallization problems, especially with high-melting couplers. Higher oil phase temperatures and longer oil solution hold times are usually required, resulting in an increased propensity for coupler decomposition during oil phase preparation. This can lead to lower and more variable coupler concentrations and the formation of photographically harmful by-products, which can cause emulsion fog and speed losses. These problems limit the number of photographically useful materials which typically have been dispersed using the direct method. It would therefore be desirable to have an improved method of preparing direct dispersions of high-melting photographically useful materials without crystallization or decomposition problems, and without causing any deleterious effects on photographic performance or physical quality.